TEGUMENTAL ORGANS 11 
such as climate, domestication, natural and artificial selection, 
influence the hairy covering. Further, the development of this 
may be in inverse ratio to the thickness of the integument, and 
particularly of the epidermis (Leydig), the hair and the epidermis 
supplementing one another in the work of protecting the body. 
This is illustrated, on the one hand, by animals which have a 
delicate epidermis and thin skin and a thick covering of wool or 
fur; and on the other by animals like the Rhinoceroses, Hippo- 
potami, some Armadillos, and Scaly Ant-Eaters, in which, while 
the epidermis is so thickened as to form a hard carapace, the 
hair is very scanty. 
I cannot leave this subject without touching upon the question of the 
origin of the Mammalia, especially as this chapter in morphology has recently 
been ably dealt with by Max Weber, who deduces reasons for taking up the 
following position. The first Mammals, as descendants from primitive scaly 
Reptiles, were covered with scales, differing from those of the Reptiles only in 
minor points. Behind the scales of the primitive Mammals there first 
appeared a few small hairs, the origin of which it is difficult to explain with 
certainty. By degrees, as a constant temperature was maintained by the 
body, the covering of hair attained a greater development and the scales 
degenerated. Scales, somewhat specialised, are still retained as a covering 
for the mammalian body in a few cases, e.g. Armadillos and Scaly Ant-Eaters. 
Among other Mammals they are found, as a rule, only on the tail and limbs. 
The recurrent arrangement of the hairs, however, due to their original 
development behind scales, has very generally persisted, and on this basis 
hairs may be considered to imply the earlier presence of scales. 
NAILS 
The nails of the fourth and fifth fingers (and especially the 
- latter) most nearly suggest the claws of the lower animals, in being 
decidedly arched from side to side. As the thumb is approached 
the nails become more and more flat, and the like is true of the 
great toe as compared with the four lesser toes. This condition 
commences with the Lemuroidea [although among the lower 
Mammalia the Squirrels, for example, bear a flattened nail upon 
the pollex]. 
On the under edge of the nail, between it and the ball of the 
finger, is found the last vestige of a structure which in the Apes 
is covered with a thickened layer of epidermis: This structure 
undergoes considerable degeneration, even during intra-uterine 
life, through the advancing development of the ball of the finger 
(Gegenbaur). 
1 This structure is most conspicuous in the Ungulata, and it is there known as the 
** frog.” 
